There have been known image forming apparatuses including developing cartridges. One of such image forming apparatuses is capable of determining whether or not the developing cartridge is attached or identifying the specification of the developing cartridge. For example, Japanese Patent Application Publication No. 2011-203362 discloses a developing cartridge including a detection gear and protrusions movable together with rotation of the detection gear. In this configuration, an image forming apparatus detects the protrusions by means of a sensor to determine whether the developing cartridge is attached.